1. Technical Field
The present invention relates generally to wireless communication systems and, more particularly, to a transmitter system and method using bandpass Delta-Sigma modulation and advanced bandpass techniques.
2. Description of Related Art
Wireless communication devices, such as cellular telephones, are widely used as a replacement for conventional telephone systems. One advantage of the wireless communication devices is their portability. The user can operate the wireless communication devices from virtually any point on earth. Since component size, weight, and power requirements of the wireless communication device can detrimentally affect portability, they are important factors that directly impact its utility.
For communication to occur, signals are transmitted from and received by components of the wireless communication devices. Transmitters, either separate or part of a transceiver, handle transmission tasks for the wireless communication device. Transmitters typically accept complex baseband signals to be transmitted. These complex baseband signals are internally generated within the wireless communication device and are made up of I and Q quadrature signal components. The transmitters subsequently perform forms of modulation, frequency up-conversion, and power amplification of the baseband signals.
Conventional transmitter architectures include a dual conversion approach and a direct conversion approach. With the dual conversion approach, modulation and up-conversion are to the desired transmission frequency performed in two steps, whereas, the direct conversion approach accomplishes modulation and up-conversion in one step. Additional conventional approaches use a translational loop or a offset phase-locked loop.
With dual conversion transmitters, a modulator performs modulation and a portion of the up-conversion, a second component, an up-converting mixer, performs the rest of the up-conversion. Dual conversion approaches are widely used with conventional transmitters of cellular telephones. When compared to direct conversion approaches, dual conversion approaches have tended to be less problematic. One serious drawback of dual conversion approaches is the need for a local oscillator associated with the modulator and a second local oscillator associated with the up-converting mixer. Use of the second local oscillator is not found with direct conversion approaches. By requiring a second local oscillator, the dual conversion approaches detrimentally increase power requirements of the wireless communication devices.
Although a second local oscillator is not needed for the up-converting mixer with the direct conversion approaches, direct conversion devices typically require the use of costly duplex filters, which have consequently hindered success of direct conversion approaches. The translational loop or the offset phase-locked loop are widely used due to their low output noise eliminating the need for costly duplex filters. However, these additional approaches also use a second local oscillator for the up-conversion portion and thus share the same disadvantage regarding power requirements as the dual conversion transmitter.
If these conventional approaches use quadrature modulation techniques, additional local oscillators are required in which a separate local oscillator and/or separate digital-to-analog converter are used for each of the I and Q quadrature signal components of the complex baseband signals, which further detrimentally affect component size, weight, and power requirements for the wireless communication devices. Furthermore, these conventional approaches suffer from imbalance errors found in typical implementations of the I and Q quadrature signal components of the complex baseband signals. Further requirements are subsequently imposed upon the conventional digital-to-analog converters to address these imbalance errors, which, unfortunately, are only partially resolved by conventional approaches.
Accordingly, there is a significant need for a system and method for a transmitter to modulate, up-convert, and power amplify complex baseband signals in wireless communication devices without the need for additional local oscillators, digital-to-analog converters, duplex filters, and other additional requirements such as the need to remedy imbalance errors between the I and Q quadrature signal components of complex baseband signals. The present invention provides this and other advantages that will become apparent from the following detailed description and accompanying figures.